Method of and apparatus for reducing width of transmission bands



Nov. 12, 1929. CARPE 1,735,037

METHOD OF AND APPARATUS FOR REDUCING WIDTH OF TRANSMISSION BANDS Filed Aug. 12, 1926 JZr/mal speedy INVENTOR l mORNEY Patented Nov. 12, 1929 UNITED STATES PATENT OFFICE ALLEN CARPE, OF NEW YORK, N. Y., ASSIGNOR TO AMERICAN TELEPHONE AND TELE- GRAPH COMPANY, A CORPORATION OF NEW YORK METHOD OF AND APPARATUS FOR REDUCING WIDTH 01f TRANSMISSION BANDS Application filed August 12, 1928. Serial No. 128,789.

This invention relates to the transmission of signals, and more particularly to methods and apparatus for reducing the Width of the frequency band required for the transmission of signaling currents, such, for example, as

speech currents, and is related to the invention of French and Zinn, Serial No. 7 55,075, filed December 10, 1924, which became Patent No. 1,671,151,-May 29, 1928.

In accordance with the present invention, it is proposed to convert each frequency of a signaling band to a lower frequency in such a manner that the converted frequencies will all bear the same ratio to the original frequencies and thus change the width of the frequency band. Having so converted the signaling current, it is proposed to transmit it over a communication channel, and upon receipt, to reconvert the message to its original form.

It should be noted that the frequency conversion of the present invention is quite distinct from the type of frequency translation involved in modulation and demodulation. Modulation and demodulation consist in simply shifting all frequencies of a band up or down in the frequency spectrum without reducing the width of the band. Frequency conversion, in accordance with the principles of the present invention, however, reduces or increases all frequencies by a certain factor, and therefore reduces or increases the width of the band by this same factor.

A system involving frequency conversion for reducing the width of the frequency band required for a message has certain advantages, such, for example, as are set forth in the application of French and Zinn, npted above.

Briefly, the system described by French and Zinn consists in interrupting a. signal train of Waves, omitting certain elements, increasing the duration of the remaining elements and thus decreasing each frequency of the signal and decreasing the width of the frequency band. The resulting wave is then transmitted overany suitable transmission channel, and on its receipt the elements are translated or re-converted into their original shorter duration and their original higher frequencies. The gaps which then appear between the successive elements may be left blank or may be filled up by repeating adjacent elements.

Ina system of the kind described, certain difficulties arise in connection with the interruption of the signal and its re-translation, and it is the purpose of this-invention to reduce or eliminate such difficulties. These latterarise in part from the fact that when a train of Waves is interrupted it is equivalent to modulating the train with the interru tion frequency and its overtones. The method which this invention discloses for reducing thedistortion due to the setting up of the modulation frequencies will be better understood as a result of the following general considerations.

Imagine a transmission channel over which a signal is passing, and associated with which there is some pick-up device, which latter may be interrupted in its function periodically. The reproduced message issuing from the pickup device will then be the original message interrupted or chopped at regular intervals corresponding to the converted frequency. If the interruption frequency is represented by a square-topped wave, alternating between zero and unity, its formula, assuming the intervals of transmission and interruption to be of equal length is i=;+asin qt+gsin3 gt+%sin 5qt+ For simplicity, we may assume the message frequency to be a pure sine wave given b the equation V=sin ft. Then the repr uced wave is given by the product of these two, as

'By the usual trigonometric transformations, it may be shown that the latter is a wave containing the frequencies ruption frequenc and its harmonics. These are frequencies w llCh represent the distortion introduced by chopping or interrupting the message.

One condition to consider is that in which the frequency g is greater than 2f. In this case, it is apparent that all the modulation frequencies will fall outside the message band and may be eliminated by suitable wave filters. If, however, we consider the current on the line in this case, it will be seen that although the individual message fragments are slowed up in transmission, there is no effective decrease in the base frequency of the signal, since the individual fragments are of less duration than the signal cycle. In effect, the original message frequency is preserved, with various discontinuities and harmonics added, whose function is to transmit the interruption frequencies, which in themselves are not necessary or desirable.

Another condition to consider is that in which the frequency of interruption g is less than 2 In this case, an actual frequency re duct-ion may be obtained, but it is evident that the'reproduced message will contain modulation products corresponding to the original frequencies plus and minus the interruption frequencies and its harmonics which will fall in whole or in part within the message band, and which can not, therefore, be eliminated by filters. It is to be noted that the distortion is not due to the interruption frequency itself, which could be eliminated by filters if sufiiciently low, but is due to side frequencies on the message frequencies which can not be so eliminated. The distortion could be minimized by making the interruption frequency low compared to the frequency 7, which, in the case of speech, would certainly mean no faster than a few cycles per second, but in such a case, this would cause important parts of the message to be lost.

A further inherent difficulty of any such system resides in the fact that the transmitted currents consist of discontinuous fragments of the original message succeeding each other without intermission, and the transmission therefore requires the reproduction of discontinuities involving transient frequencies on the line. These transient frequencies will be suppressed to some extent in practice, but in order that this may be done without distorting the message, the interruption frequency must obviously be as low as possible.

In this invention, I propose to still keep the interruption frequency low compared with the transmitted signal frequency. so that in each interrupted element there will be a plurality of the signaling waves, and so that the transient discontinuities will be infrequent compared with the transmitted frequency, but, at the same time, I propose to keep the interruption frequency sufiiciently removed from the signaling frequencies so that the modulation side frequencies will not fall within the signaling band. This I accomplish in the case of a normal telephone current by using this latter to modulate a carrier frequency which is high compared to the message frequencies and then interrupting this modulated wave at an intermediate frequency which is low compared to the carrier frequency, but itself is more than twice the highest frequency contained in the original message. As a result, the modulation frequencies existing in the reproduced message will be of such frequencies that they maybe eliminate by suitable filters.

The invention will be better understood by reference to the accompanying drawing, in which Figure 1 illustrates a circuit suitable for the transmitting end of such a system as I propose; and Fig. 2 represents the receiving end of such a system. Figs. 3 to 5 illustrate forms of devices which may be used for conversion at the transmitting and receiving ends.

Referring more specifically to Fig. 1, a line L is shown, over which a speech wave may enter. This speech wave will be a complex one made up of a large number of frequencies varying from about 100 cycles to 5,000 cycles. For simplicity in description, this comparatively broad band of frequencies will be represented throughout by the letter f, and whenever used it is to be understood that the letter f does not stand for any single frequency, but for any and all of the frequencies in the speech range and included in the incoming message. This speech signal is impressed upon a modulator M, along with a carrier frequency 29 generated by an oscillator O. The modulation products are impressed upon a band pass filter so adjusted as to pass one side band only, such as that one extending from the frequencies p to 19+). The wave is now allowed to pass a transforming device 9 which adapts it to be propagated over a channel or path 10 of a type such that the wave velocity is the same for all frequencies, and preferably of such a type that the velocity is relatively low. A convenient form which this path 10 may take wouldbe that illustrated in Figs. 12? and 13 of the application to French and Zinn. Another convenient form which it may take is illustrated in Figs. 3 and, 5, to be described below. Preferably, this path 10 would be circular in form, and rotating about the axis of the circle there would be a pick-up device 11 suitably connected by contacts to the lineL The wave may be considered as progressing along the path 10 with a definite velocity V, and it is evident that if the pick-up device 11 is stationary, the wave will be transmitted without Ill change from the line L to the line L,. If,

line L so fastas it comes on to the path 10, and there will thus be an increase in the duration of any element of signal carried through, and a corres onding decrease in the frequencies and in t e width of the frequency band. The change which will occur obviously will depend upon the relative velocity of the pick-up device and of the wave of the path 10. For simplicity in the description, and

of the message wave which would occupy half the circumference of the path 10, and that this segment will now be spread over a length corresponding to one complete circumference of the path 10. The duration of this element, therefore, has been doubled, and each frequency in it will have been halved.

It is to be observed that when the element 11 has made one complete revolution, it skips to the beginning of the path 10, and the portion of the message Wl11Cl1 then occupies the path 10 will be omitted so far as transmission to L is concerned. As a result, there will appear upon this line L alternate elements of the original message, the frequency 0+ f within each element having been re- 3 and the elements following duced to each other without intermission, as explained above.

The converted signal as thus produced may now be transmitted to the remote receiving station E of Fig. 2, in which virtually the reverse process is followed in order to reproduce the signal in normal form. To this end, the incoming signal of the frequency mi 2 vice 19, similar to 9 and which adapts the wave to be propagated with a velocity V which may be the same as a velocity V of Fig. 1, over a path 20 similar to the path 10 but preferably comprising a semi-circle only. Associated with said path is a pick-up device 21 adapted to rotate about the center of the path 20 and connected by suitable contacts with the -line L In this case, the device 21 rotates in the opposite direction to that in which the wave is propagated over the path 20, and with a velocity V numerically equal to that of the velocity of propagation. Consideration of this device will make it apparent that any element of signal picked up is allowed to enter a transforming deby 21 will be transferred to the line L as an'element of one-half the duration of the time required for it to enter upon the path 20. This obviously entails also a doubling of each and every frequency component. The signal currents ISSlllIW from this device will thus correspond to t e original modulated message at point 11 in Fig. 1, but with alternate elements omitted. These omissions will follow each other at a frequency higher than the original message frequency but substantially lower, as explained above, than the frequency of the modulated message. These interruptions or omissions are represented on the line L by modulation frequencies, the fact that these are present being indicated by the legend (12+ finq). These modulation frequencies may be partly eliminated by a band filter 23, this filter being adjusted to pass the band p to 12+ as indicated by the single reference 12+ it being understood-that f takes on substanstantially all values from zero to 5,000. In order to hear the messa e, it is now necessary to demodulate it, and or this purpose a demodulator DM is shown, supplied by the incoming signal and by a carrier wave of frequency p from the oscillator O.

The out put of-this demodulator will consist of the original message with interruptions (so far as not already eliminated by filter 23) at a frequency above the message frequency and these interruption frequencies may therefore befurther eliminated by means of a band pass or low pass filter 25, adjusted to trans mit only the frequencies I, this being the normal speech message taken in at the transmitting station.

If the modulator and filter of Fig. 1 is not of such a form as to eliminate the carrier component, it will not be necessary to have the oscillator O" of Fig. 2. In practice, how ever, the modulator of Fig. 1 may be of the balanced type thus eliminating the carrier or unmodulated component ofthe wave and then 0 will be necessary. The demodulator of Fig. 2 may also in that event be of the balanced type.

In order to avoid distortion, it is desirable that the picking off of the high fre- The velocity of propagation over paths 2O and 10, and the speed of rotation of arms 21 and 11 should be so related as to reproduce the message segment-s at their original frequency and the arms should furthermore be in phase at the beginning of each segment, i. e., a form of synchronism should be maintained and any suitable mechanism may be used for this. This means that if arms 21 and 11 are to rotate with equal velocity, the velocity of the wave in 20 should be one half that in 10.

Instead of the path 20 being semi-circular,

it may be circular and then a consideration of the figure will show that the gaps of the resultant message already described will be filled by repetition of the adjacent portions of the message. In this case, the volume of the message receivedwill be increased but there will be moredistortion.

\Vhile it is to be understood that any suitable form of device may be used in place of the paths 10 and 20, and the pick-up devices 11 and 21, I have shown one form which they may taken on in Figs. 3 and 4. Fig. 3, for example, illustrates a device which might be inserted in the circuit of Fig. 1 between the portions 11 and 22. Referring to Fig. 3, the frequency 12+ 7 may be impressed upon a magnetic winding 30 adjacent to a continuously moving steel tape or wire 31. The incoming message is thus recorded magnetically on this wire which travels with a constant velocity V around a circular path corresponding to the path 10 of Fig. 1. As the wire so travels, it is equivalent to the propagation of the signal wave in that path with the'vclocity V. A magnetic pick-up or reproducer 32 is mounted on an arm and adapted to rotate adjacent to the magnetized wire and with a velocity 5 in the same direction as that of the wire, thus yielding the results described in connection with Fig. 1.

As a receiving device, Fig. 4 may be substituted for the element shown in Fig. 2 between 33 and 44. In this case, the converted signal of frequency 2323 is received on a recording magnet 40, and a magnetic record made on a steel tape or wire 41. This steel tape travels in a manner similar to that described in connection with Fig. 3, but for a half circle only, after which it departs substantially. Adjacent to semi-circular portion and rotating in the opposite direction is a reproducer magnet 42 traveling with a velocity equal to that of the steel tape. As a result, there will be a doubling of the incoming frequency, all as described in connection with Fig. 2. In these figures, devices 34 and 44 are shown to restore the wire to normal condition. These are shown as supplied from a D. C. source but an A. C. source may be used if preferred.

Instead of a moving magnetic tape, many other forms of moving records may be used,

such as a phonograph record-or a photographic film adapted for recording and reproduc tion. Still another form the converter may take is shown in Fig. 5 in which the incoming transmission line is continued as a transmission network represented as a pair of conductors forming two concentric circles or two parallel circles with a suitable terminating network 51 which does not give any reflected wave. At the receiving station this path would be a semi-circle.

as follows.

What is claimed is:

1. The method of reducing the width of a hand of signaling frequencies during transmission, which comprises modulating the signal train to a carrier frequency, reducing the frequency of successive time elements of the resulting train of waves element by element, transmitting separated portions of the converted wave, translating the received converted wave train portion by portion to a higher frequency, and demodulating the wave to normal signal frequency.

2. The method of reducing the width of a band ofsignaling frequencies during transmission, which comprises modulating the signal train to a carrier frequency, reducing the frequency of successive time elements of the resulting train of waves element by element, transmitting separated portions of the converted wave in such time relation to each other that no blank intervals will exist, translating the received converted wave train portion by portion to a higher frequency, and demodulating the wave to normal signal frea queney.

3. The method of reducing the width of a band of signaling frequencies during transmission, which comprises modulating the signal train to a carrier frequency, interrupting the signal frequency into elements at a frequency substantially different from the carrier frequency, suppressing certain elements, expanding the time duration of the adjacent elements to extend into the time of sup pressed elements, transmitting the separated portions, translating the received converted wave train portion by portion to its original shorter duration, and demodulating the wave to normal frequency.

4. The method of reducing the width of a band of signaling frequencies during transmission, which comprises modulating the signal train to a carrier frequency, interrupting the signal frequency into elements at a frequency low compared to the carrier frequency, suppressing certain elements, expanding the time duration of the adjacent elements to extend into the time of suppressed elements, transmitting the separated portions, translating the received converted wave train portion by portion to its original shorter duration, and demodulating the wave to normal frequency.

5. The method of reducing the width of a band of signaling frequencies during transmission, which comprises modulating the signal train to a carrier frequency, interrupting the signal frequency into elements at a frequency intermediate the carrier and the signal frequency, suppressing certain elements, expanding the time duration of the adjacent elements to extend-into the time of suppressed elements, transmitting the separated portions, translating the received converted wave train portion by portion to its original shorter duration, and demodulating the wave to normal frequency.

(3. The method of reducing the widthof a hand of signaling frequencies during trans mission, which comprises modulating the signal train to a carrier frequency, interrupting the signal frequency into elements at a fre quency intermediate the carrier frcqueneyand the signal frequency, suppressing certain elements, expanding the time duration of the adjacent elements to occupy the time of the suppressed elements and in such time relation to. each other that no blank intervals will exist, transmitting the separated portions, translatin the received converted wave train portion hy portion to its original shorter duration, and demodulating the wave to normal signal frequency.

7. In a transmission system. a transmitting station, means thereat for reducing the width of a hand of signaling frequencies during transmission comprising a source of signals to be transmitted, means for modulating the signal to a carrier frequency, means for interrupting the modulated carrier wave into elements and narrowing the frequency range.

8. In a transmission system, a transmitting station, means for reducing the width of a hand of signaling frequencies comprising a source of signals to be transmitted, means for modulating the signal to a carrier frequency, and means for interrupting the modulated carrier wave at a frequency substantially below the carrier frequency.

9. In a transmission system, a transmitting station, means for reducing the width of a hand of signaling frequencies comprising a source of signals to be transmitted, means for modulatingthe signal to a carrier frequency and for suppressing all but one side band, and means for interrupting the said side hand at a frequency substantially below the carrier frequency.

10. In a transmission system, a transmitting station, means for reducing the width of a hand of signaling frequencies comprising a source of signals to be transmitted, means for moduating the signal to a carrier fre quency, means for interrupting the modulated carrier wave at a frequency substantially be' low the carrier frequency, a receiving station, a frequency expanding means associated therewith for expanding the received wave train to normal frequency width, and means for demodulating the resultant wave to normal signaling frequency.

11. In a transmission system, a transmitting station, means for reducing the. width of a hand of signaling frequencies comprising a source of signals to be transmitted, means for modulating the signal to a carrier frequency, means for interrupting the modulated 'arrier wave at a frequency substantially below the carrier frequency, a receiving station, a frequency expanding means associated therewith for expanding the received wave train to normal frequency width, means for demodulating the resultant wave to normal signaling frequency, and wave filters at the receiving station for eliminating the interrupter and expander modulation frequencies.

12. In a signaling system, means for reducing the width of a signal frequency band, comprising a moving record, means for recording the signal thereon, and a moving pick-up device adjacent to said record having a uniform velocity different from the velocity of the record and moving always in the same direction as the record.

13. In a signaling system, means for contracting the width of a signal frequency band, comprising a moving iron tape or wire, a magnetic recording means adjacent thereto, a magnetic pickup device adjacent to said tape or wire and having a uniform velocity diflerent from the velocity of'the tape or wire and moving always in the same direction as the tape or wire.

14. In a transmission system, a transmitting station, means thereat for reducing the width of a band of signal frequencies during transmission, comprising a source of signals to be transmitted, means for modulating the signal to a carrier frequency, and means for suppressing alternate elements of the modulated carrier wave and expanding the time duration of the other elements.

15. In a transmission system, a transmit-- ting station, means thereat for reducing the width of a band of signal frequencies during transmission, comprising a source of signals to be transmitted, means for modulating the signal to a carrier frequency, means for suppressing alternate elements of the modulated carrier wave and expanding the time duration of the other elements, a receiving station, and means at that station for compressing the received expanded elements to their normal duration.

16. In a transmission system, a transmitting station, means thereat for reducing the width of a band of signal frequencies during transmission, comprising a source of signals to be transmitted, means for modulating the signal to a carrier frequency, means for suppressing alternate elements of the modulated carrier Wave and expanding the time duration of the other elements, a receiving station, means at that station for compressing the received expanded elements to their normal duration and for repeating each element to fill up the time spaces.

17. In a signal transmission system, a

. transmitting. station, means thereat for reducing the Width of a band and signal frequencies during transmission, comprising a source of signals to be transmitted, and means for suppressing alternate elements of the signal and expanding the time duration of the other elements.

18. In a transmission system, a transmitting station, means thereat for reducing the Width of a hand of signal frequencies during transmission, comprising a source of signals to he transmitted, means for suppressingalternate elements oi the signal train and e, panding the time duration of the other elements, a receiving station, and means station for compressing: the recc paneled elements to their normal duration.

19. In a signal transmission system, a transmitting station, means thereat for reducing the Width of a hand of signal frequencies during transmission, comprising a source of signals to he transmitted, means for suppressing alternate elements of the signal train and expanding the timed duration of the other elements, a receiving station, means at that station for compressing the received expanded elements to their normal duration and for repeating each element to fill up the time spaces.

20, In a signal system comprising a transniitting and a receiving station, means at the transmitting station for contracting the Width of the signal frequency band comprismg a moving record, means for recording the a signal thereon and a moving pick-up device adiacent to said record having a velocity difterent from the velocity of the record and moving always in the same direction as the record, and means at the receiving station for expanding the Width of the received frequency hand, comprising a moving record, means for recordings; the received signal thereon, and a moving pick-up device adiacent to said record having a uniform velocity and moving always in the opposite direction to the record.

in testimony whereof, I have signed my I name to this specification this 11th day of August, 1926.

ALLEN CARPE. 

